System and method of dynamic regulation of real power to a load

ABSTRACT

A system and method for controlling an AC motor drive includes a control system programmed with an energy algorithm configured to optimize operation of the motor drive. Specifically, the control system input an initial voltage-frequency command to the AC motor drive based on an initial voltage/frequency (V/Hz) curve, receives a real-time output of the AC motor drive generated according to the initial voltage-frequency command, and feedback a plurality of modified voltage-frequency commands to the AC motor drive, each of the plurality of modified voltage-frequency commands comprising a deviation from the initial V/Hz curve. The control system also determines a real-time value of the motor parameter corresponding to each of the plurality of modified voltage-frequency commands, and feeds back a modified voltage-frequency command to the AC motor drive so that the real-time value of the motor parameter is within a motor parameter tolerance range.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a non-provisional of, and claims priority toU.S. Provisional Application Ser. No. 61/186,287 filed Jun. 11, 2009,which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates generally to AC motors and, moreparticularly, to a system and method for reducing real power usage ofopen-loop AC motor drive systems (which can include at least a motordrive, a motor, and a connected load).

Open-loop AC motor drives are typically programmed to operate based on avoltage/frequency (V/Hz) operating curve. Because the V/Hz settings ofan open-loop motor drive is typically adjustable only while the drive isnot in operation and/or the actual motor loading conditions are notknown at time of drive commissioning, the V/Hz operating curve istypically a pre-set and static curve that is programmed during theinitial installation and setup of the drive. The motor drive may beprogrammed with a first order or linear V/Hz operating curve to maintaina constant ratio between the voltage and frequency applied to the motor.Such an operating curve maintains a constant flux in the air-gap and,therefore, generates constant torque in the motor. Alternatively, asecond order V/Hz curve may be selected, where the output torque isapproximately proportional to the square of the motor speed. Some motordrives also provide preset user-programmable V/Hz settings to meet therequirement for special applications. Once programmed, the motor drivetypically operates based on the pre-set operating curve during thelifetime of the drive unless an operator changes the drive setting at alater time.

Some drives today have energy saving options, such as “FluxMinimization” or “Flux Optimization.” These options are often designedto dynamically seek a minimal current or flux in the motor, but not theoverall motor input power. These options typically select a motorvoltage which is between the linear V/Hz setting and the quadratic V/Hzsetting.

Typically, the linear V/Hz curve, commonly referred as constant V/Hzcurve, is the default setting of the majority of open-loop motor drives.While the linear V/Hz curve settings for a given motor drive aretypically programmed to provide constant torque to the load, suchconstant torque output is not needed for many variable torqueapplications. Therefore, the V/Hz curve settings often result in wastedenergy and inefficient operation of the motor drive system, especiallythe motor and its connected load.

Furthermore, because the V/Hz curve settings are static, the motor driveoperates independently of any changes in operating conditions. Althoughsome motor drives may allow a user to alter the V/Hz curve by, forexample, adjusting the start, middle, and/or end points of the V/Hzcurve, such programming cannot be performed automatically by the driveitself and may be completed only when the motor is shutdown. Thistypically requires a skilled technician who has a thorough understandingof the both motor drive and the loading profile of the specific motorand load application.

Another reason why an operator commissioning a drive typically selects alinear V/Hz curve is that the linear V/Hz curve provides constant torqueand minimizes the risk that the actual torque using a non-linear V/Hzcurve (e.g., second order curve) may not be sufficient to meet the loadtorque demand for variable torque applications. This is particularlytrue if the operator commissioning the drive does not fully understandthe load profile of the motor application where the drive is installed.For example, in waste water treatment plants, the pump motors are oftendesigned to meet the peak demand when heavy rain occurs in the summer.During the majority of the year, however, the load percentage of thepump motor can be very low. Using a second order V/Hz curve in thisapplication may be risky because the second order V/Hz curve may notprovide enough torque when a heavy rain event occurs. Also, the operatorwho commissions the drive is usually not the same person in a plant whois responsible for monitoring and controlling energy savings. Thus,there may be little incentive for the operator who commissions the driveto select a different V/Hz curve setting for energy savings over thestandard linear V/Hz curve settings.

It would therefore be desirable to design an apparatus and method fordynamically adjusting the V/Hz operating curve of an open-loop AC motordrive during motor operation, so that the drive itself can determine anoptimal voltage and frequency applied to the motor to minimize the motorinput real power and achieve additional energy savings while maintainingstable motor operating conditions.

BRIEF DESCRIPTION OF THE INVENTION

The present invention provides a system and method for controlling an ACmotor drive connected to a load that overcomes the aforementioneddrawbacks.

In accordance with one aspect of the invention, a control system isprogrammed to input an initial voltage-frequency command to the AC motordrive based on an initial voltage/hertz (V/Hz) curve, receive areal-time output of the AC motor drive generated according to theinitial voltage-frequency command, and feedback a plurality of modifiedvoltage-frequency commands to the AC motor drive, each of the pluralityof modified voltage-frequency commands comprising a deviation from theinitial V/Hz curve. The control system is also programmed to determine areal-time value of the motor parameter corresponding to each of theplurality of modified voltage-frequency commands and feedback a modifiedvoltage-frequency command to the AC motor drive so that the real-timevalue of the motor parameter is within a motor parameter tolerancerange.

In accordance with another aspect of the invention, a method forcontrolling a motor drive output includes operating a motor driveaccording to a static voltage-frequency profile to generate an outputpower and determining a value of a specified motor parametercorresponding to the static voltage-frequency profile. The method alsoincludes determining a tolerance zone of the specified motor parameter,selectively modifying the static voltage-frequency profile duringoperation of the motor drive to generate a reduced motor input power,and operating the motor drive at a voltage-frequency settingcorresponding to a value of the specified motor parameter within thetolerance zone.

In accordance with yet another aspect of the invention, a motor drive isconfigured to supply power to a load. The motor drive includes aninverter designed to provide power to the load and a controlleroperationally connected to control operation of the inverter. Thecontroller is configured to cause the inverter to adjust avoltage-frequency setting during operation of the motor drive to causethe inverter to operate according to each of a plurality of variedvoltage-frequency settings and monitor a real-time value of a motorparameter corresponding to each voltage-frequency setting. Thecontroller is also configured to identify a tolerance zone for the motorparameter and cause the inverter to operate according to thevoltage-frequency setting such that the motor parameter is within thetolerance zone.

Various other features and advantages of the present invention will bemade apparent from the following detailed description and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate preferred embodiments presently contemplated forcarrying out the invention.

In the drawings:

FIG. 1 is schematic of a control system including a motor drive systemaccording to one aspect of the invention.

FIG. 2 is a schematic of a control system including a motor drive systemaccording to another aspect of the invention.

FIG. 3 is a schematic of a control system including a motor drive systemaccording to yet another aspect of the invention.

FIG. 4 is a flow chart setting forth exemplary steps of a motor drivecontrol technique that may be implemented in any of the motor drivesystems of FIGS. 1-3 according to embodiments of the invention.

FIG. 5 is a series of exemplary graphs of real-time motor parameters forthe motor drive control technique of FIG. 4.

FIG. 6 is an exemplary graph illustrating energy savings for a givenmotor drive operated according to the motor drive control technique ofFIG. 4 according to an embodiment of the invention.

FIG. 7 is an exemplary graph illustrating energy savings for a givenmotor drive operated according to the motor drive control technique ofFIG. 4 according to an embodiment of the invention.

FIGS. 8-10 are a series of exemplary graphs illustrating a comparison ofa number of motor drive control techniques.

DETAILED DESCRIPTION OF THE INVENTION

Several embodiments of the invention are set forth that relate to asystem and method of controlling an AC motor drive connected to a loadthat overcomes the aforementioned drawbacks. These embodiments of theinvention are directed to an energy-optimizing control system foropen-loop motor drives encompassing a plurality of structures andcontrol schemes.

A general structure of a motor drive system 10 and an associated ACmotor drive 12 is shown in FIG. 1. The motor drive 12 may be configured,for example, as an adjustable speed drive designed to receive athree-phase AC power input 14 a-14 c, rectify the AC input, and performa DC/AC conversion of the rectified segment into a three-phasealternating voltage of variable frequency and amplitude that is suppliedto a load. According to one embodiment, a drive control unit 16 may beintegrated within motor drive 12 and function as part of the internallogic of motor drive 12. Alternatively, drive control unit 16 may beembodied in an external module distinct from motor drive 12, and receivedata therefrom (e.g., voltage and/or current signals), as described inmore detail with respect to FIGS. 2 and 3.

Referring to FIG. 1, in an exemplary embodiment, motor drive 12 includesa drive power block unit 18, which may, for example, contain auncontrollable or controllable rectification unit 20 (uncontrolled AC toDC), a filtering inductor 22, a DC bus capacitor 24, and a pulse widthmodulation (PWM) inverter 26 (DC to controlled AC). Alternatively, drivepower block unit 18 may be provided without such a rectification unitsuch that the DC bus is directly connected to the inverter. A drivepower block unit may be provided without a rectification unit whenapplied to an uninterruptible power supply (UPS), for example.

Motor drive 12 receives the three-phase AC input 14 a-14 c, which is fedto the rectification unit 20. The rectification unit 20 converts the ACpower input to a DC power such that a DC bus voltage is present betweenrectification unit 20 and PWM inverter 26, which inverts and conditionsthe DC power to a controlled AC power for transmission to an AC motor28. PWM inverter 26 includes a plurality of switches (not shown) and isconfigured to operate according to a PWM control scheme, such as, forexample, a Space Vector Modulation (SVM) control scheme or aSinusoidal-Triangle PWM control scheme, to control the plurality ofswitches, thereby producing the controlled AC power output. According toan exemplary embodiment, PWM inverter 26 is configured to operateaccording to a SVM control scheme.

Drive control unit 16 operates to generate the SVM control scheme forthe PWM inverter 26. More specifically, the SVM control scheme for thePWM inverter 26 is generated by motor drive system 10 based on avoltage-frequency (V/Hz) setting or command (i.e., V/Hz profile orcurve) used for operating motor drive 12. According to an exemplaryembodiment of the invention, motor drive system 10 is programmed todynamically adjust the voltage and frequency applied to motor 28 basedon motor or load demand, which in effect dynamically adjusts the shapeor profile of the pre-set V/Hz curve (and the associated SVM controlscheme) internal to motor drive 12.

Motor drive 12 further includes a drive user interface 30 or drivecontrol panel, configured to input motor parameters 32 and output afrequency reference 34, a boost voltage 36, which is which is used toproduce starting torque to accelerate motor from zero speed, and motornameplate information (NPI) 38. User interface 30 is also used todisplay a list of motor operating parameters, such as, for example motoroutput voltage (rms), motor current (rms), motor input power, speed,torque, etc., to the user for monitoring purposes.

As shown in FIG. 1, drive control unit 16 includes a control algorithmmodule 40, an existing or preset V/Hz curve 42, a signal generator 44for generating the SVM control, and a motor parameter calculator 46.Drive control unit 16 functions to receive an output from drive powerblock unit 18, determine and monitor motor parameter(s), and determinean optimal voltage and frequency based on the determined motorparameter(s) to generate a SVM control scheme for operating motor drive12.

According to an embodiment of the invention, drive control unit 16receives DC bus voltage signal 48 and motor input current signals 50from drive power block unit 18. Motor input voltages are calculatedusing DC bus voltage signal 48 and PWM switching signals 56. Signals 50and motor input voltage signals may be obtained from the AC power outputof the motor drive 12, such as by way of wired or wireless sensors thattransmit the real-time voltage signals 48 and real-time current signals50 thereto. Alternatively, motor parameter calculator 46 may receivesignals indicating motor speed from a speed sensor or estimatorintegrated in motor drive 12. Drive control unit 16 may also receiveboost voltage signal 36, which is used to produce starting torque toaccelerate AC motor 28 from zero speed, and a speed reference signal 52.Based on received signals 36, 48, 50, drive control unit 16 transmits aseries of switching signals or switching commands 54 to PWM inverter 26,thereby forming a SVM control scheme.

In operation, drive control unit 16 of motor drive system 10 operates ina default mode/setting upon a start-up or reset of the motor drive 12.Operating at the default setting, drive control unit 16 monitors DC busvoltage signal 48 and current signals 50 from sensors, determines anoperating point of existing V/Hz curve block 42 based on DC bus voltagesignal 48 and current signals 50 and transmits default switchingcommands 54 to PWM inverter 26 based on the pre-set operating point.According to one embodiment, drive control unit 16 receives a frequency(or speed) command from an input device (not shown) in order to generatea frequency command and a voltage magnitude command. The voltagemagnitude command is given by a function of the frequency command,typically referred to as a V/Hz curve. Drive control unit 16 generates athree phase voltage command based on the frequency command, which isused to control switching of an array of switches in PWM inverter 26.Specifically, signal generator 44 receives a voltage command 56 and afrequency command 58 from existing V/Hz curve 42 and generates six PWMsignals to control six corresponding switches in PWM inverter 26. Inother words, motor drive system 10 transmits voltage-frequency commandsaccording to the static pre-set V/Hz profile in default mode.

Upon an initial operation in default mode, drive control unit 16 thentransitions to operate in an energy-optimizing mode wherein controlalgorithm module 40 bypasses existing V/Hz curve block 42 and receivesfrequency reference 34, boost voltage signal 36, and NPI 38 from driveuser interface 30 as inputs. Control algorithm module 40 also receivesestimated or calculated motor parameters 60 from motor parametercalculator 46. In the energy-optimizing mode, control algorithm module40 uses the received DC bus voltage signal 48 and/or current signals 50and NPI 38 to calculate or estimate selected reference real-time motorparameters. In one embodiment of the invention, the real-time motorparameters may be an average motor rms voltage, an average motor rmscurrent, an instantaneous motor input power factor, a motor efficiency,or a motor slip (or speed). Control algorithm module 40 then determinesif the optimal operation is achieved by determining if any one of thedetermined real-time motor parameters reaches its pre-defined tolerancezone, as described in more detail with respect to FIG. 4. If the optimaloperation is not attained, the algorithm adjusts the voltage commandwith a fixed or variable step and keeps the same frequency command.Otherwise, the algorithm maintains the same voltage and frequencycommand until a new frequency reference is detected or an undesirableoperation or an unstable motor operation is detected. Control algorithmmodule 40 transmits the determined voltage command 62 and frequencycommand 64 to signal generator 44. Using voltage and frequency commands62, 64 received from control algorithm module 40, signal generator 44transmits switching signal 54 to drive power block unit 18. Responsivethereto, drive power block unit 18 synthesizes AC voltage waveforms witha fixed frequency and amplitude for delivery to AC motor 28.

In energy-optimizing mode, drive control unit 16 is configured tocontinuously monitor motor drive system 10 and transmit a series ofmodified switching commands 54 to PWM inverter 26. Specifically, drivecontrol unit 16 calculates motor input voltages using DC bus voltagesignal 48 and PWM switching signals 56 and receives current signals 50from sensors. Drive control unit 16 calculates or estimates one or morereal-time motor parameters from the calculated motor input voltages,current signals 50, and NPI 38. Control module 40 also determines ifsystem instability, an abrupt load change, or an undesirable operationis detected, as described in detail below. Control module 40 furtherdetermines whether an optimal operation condition has been reached basedon the one or more real-time motor parameters. If control module 40 doesnot detect an optimal operation condition, system instability, an abruptload change, or an undesirable operation, control module 40 thengenerates incremented (or decremented) frequency command 64 and/orincremented voltage (or decremented) command 62 for each executionperiod and transmits the incremented voltage-frequency commands 62, 64to signal generator 44. For example, control module 40 may increment (ordecrement) only one command 62, 64, for example frequency command 64,while maintaining the previous voltage command 62, and transmitincremented frequency command 64 and non-incremented voltage command 62to signal generator 44. Alternatively, control module 40 may increment(or decrement) both frequency command 64 and voltage command 62 andtransmit both incremented commands 62, 64 to signal generator 44. Usingthe incremented (or decremented) voltage-frequency commands, signalgenerator 44 modifies switching commands 54 transmitted to PWM inverter26 such that the real power input to AC motor 28 in energy-optimizingmode is lower than the real power input using the original static V/Hzcurve setting in default mode.

According to an exemplary embodiment, the pre-defined tolerance zone ofa motor reference parameter is defined as a small range around apre-defined value, such as, for example, its maximum and minimum value,its rated or nominal value, or any specific value defined by the user.The trend of this real-time motor parameter is monitored to determine ifthe real-time value of this motor reference parameter falls within itspre-defined tolerance zone, e.g., if the real-time value of this motorparameter is close enough to its maximum or minimum value. If thealgorithm determines that the motor parameter is within its tolerancezone, then optimal operation is attained. The algorithm maintains thesame voltage and frequency commands until a new frequency reference oran unstable motor operation or an undesirable operation is detected.According to this optimally maintained voltage-frequency command, thesignal generator 44 is caused to generate corresponding switchingcommands 54 (i.e., a SVM control command) to cause the motor drive tooperate at an optimal operation point at which real power input to ACmotor 28 is most efficiently utilized.

In energy-optimizing mode, drive control unit 16 is also configured tocontinuously monitor for an abrupt load change, system instability,and/or undesirable operation. If system instability and/or undesirableoperation is confirmed, drive control unit 16 transmits a series ofmodified switching commands switching commands 54 to PWM inverter 26.During energy-optimizing mode, the system may become unstable due to,for example, abrupt changes in the load or if motor slip (or speed) goesbeyond a slip (or speed) boundary. A system instability condition may bedetermined by monitoring for abrupt changes in the value orrate-of-change of motor current, power factor, or speed (or slip)signals or by monitoring the trend of real-time motor parameter(s), asdiscussed in detail below. By comparing the real-time motor parameter(s)and the ascertained trend of the real-time motor parameter(s) with theirpre-defined boundaries, control module 40 determines if systeminstability or an undesirable operation is detected. If such systeminstability or an undesirable operation is detected, control module 40may transmit modified voltage-frequency commands to signal generator 44to attempt to regain system stability. Alternatively, control module 40may enters a “master reset routine” to reset control of motor drive tothe default mode within one or more execution periods and increment (ordecrement) voltage command 62 to the original pre-set V/Hz curve orlinear V/Hz curve, while transmitting the same frequency command 64 tosignal generator 44 to reacquire and maintain system stability until theoperating point of the load is considered stable. According to oneembodiment, frequency command 64 may be held constant until a user or anexternal process controller demands a change in frequency reference 34.

Drive control unit 16 may also monitor real-time motor parameters forapplication-specific boundary conditions, which may be pre-set by anoperator to indicate a real-time system condition that may not indicatean unstable or undesirable system condition, but nonetheless beundesirable for the specific application. For example, drive controlunit 16 may monitor for a minimum voltage boundary, a maximum voltageboundary, a maximum current boundary, a maximum slip (minimum speed)boundary, a minimum power factor boundary, a maximum torque boundary, ora maximum motor temperature boundary.

During energy-optimizing mode, however, a reduced voltage command maycause a decrease in motor speed. Therefore, drive control unit 16 may befurther programmed to monitor motor speed and increase the frequencycommand 64 such that the motor operates at the desired speed whileoperating in energy-optimizing mode according to a “slip compensationmode” such that a motor shaft speed is held constant at the synchronousspeed of the frequency reference f_(ref) 34. Beneficially, the V/Hzcurve (either linear, quadratic, or other similar settings (e.g., a fluxminimization setting)) can thus be adjusted to optimize the V/Hz ratioso that minimal torque is developed in the motor to meet a load torquedemand, as compared to a pre-set V/Hz curve where the torque developedis constant.

Assuming the actual motor fundamental frequency f₁ closely matches thedrive frequency command f_(cmd) 64 (i.e., f₁=f_(cmd)), the motorsynchronous speed ω_(syn) can be calculated according to:

$\begin{matrix}{{\omega_{syn} = {\frac{120 \times f_{1}}{p} = \frac{120 \times f_{cmd}}{p}}},} & \left( {{Eqn}.\mspace{14mu} 1} \right)\end{matrix}$where p is the number of poles of motor 28. As shown in FIG. 6, due tothe actual loading condition of the load (i.e., the shape of the loadcharacteristic curve), the actual motor shaft speed CO, when the drivefrequency command 64 is f_(cmd) (thus the motor fundamental frequency isf₁=f_(cmd)) is always slightly lower than the synchronous speed ω_(syn).The percentage difference between the synchronous speed ω_(syn) andmotor shaft speed is defined according to:

$\begin{matrix}{{s = \frac{\omega_{syn} - \omega_{r}}{\omega_{syn}}},} & \left( {{Eqn}.\mspace{14mu} 2} \right)\end{matrix}$where s is motor slip. Therefore, in order to compensate the speed dropdue to load, the drive frequency command 64 can be set slightly higherthan frequency reference 34 so that the actual motor shaft speed equalsto the synchronous speed of the original frequency reference 34. This isthe “slip compensation mode.”

The slip compensation mode may be used when the user or drive outer-loopcontroller expects the motor shaft speed to match the synchronous speedof the frequency reference 34 (in this case, frequency reference 34 isgiven as essentially a “speed reference.”) For example, for a 4-poleinduction motor, when a user sets frequency reference 34 at 40 Hz in thedrive user interface, the user often expects the motor to operate at thesynchronous speed of 40 Hz (i.e., 2400 rpm). However, if the drive sendsa frequency command 62 of 40 Hz, the actual motor speed will be slightlylower than 2400 rpm due to actual loading condition (according to FIG.6), for instance, at 2375 rpm. Using Eqn. 2, the slip of the motor maybe calculated as

$\left( \frac{2400 - 2375}{2400} \right).$In order to operate the motor at a user expected 2400 rpm, the drivesends a frequency command 62, which slightly greater than the frequencyreference 34 of 40 Hz, for example 40.2 Hz. Under this higher frequencythe motor shaft speed is 2400 rpm that matches the user's “speedreference.”

Still referring to FIG. 1, during energy-optimizing mode existing V/Hzcurve block 42 may be dealt with in several ways according toembodiments of the invention. According to one embodiment, controlalgorithm module 40 may be implemented in drive application software,while the existing V/Hz curve block 42 may be implemented in drivefirmware. In such an embodiment, existing V/Hz curve block 42 maycontinue to produce voltage and frequency commands, but such commandsmay not pass to signal generator 44. Alternatively, both controlalgorithm module 40 and existing V/Hz curve block 42 may be implementedin drive firmware. In this case, existing V/Hz curve block 42 may bedisabled or removed.

Referring now to FIG. 2, the general structure of a motor drive system66 is shown according to an embodiment of the invention. Motor drivesystem 66 includes an AC motor drive 68, a drive user interface 70, anda standalone external control module 72. A drive control unit 74included within motor drive system 66 comprises a static V/Hz curveblock 76, a signal generator 78, and a motor parameter calculator 80,which receives DC bus voltage signal 82 and motor current signals 84from a drive power block unit 86.

Control module 72 includes an energy control algorithm module 88 as aseparate hardware module external to the existing hardware of motordrive 68 and may be installed in an existing motor drive and exchangedata through existing drive communications, such as, for example,ModBus, Device Net, Ethernet, and the like. Control module 72 uses a setof voltage sensors 90 to measure the three phase line-to-line voltagesof a motor 92. Control module 72 also includes a set of current sensors94 to measure the three phase currents of motor 92. Where no neutralpoint is available, control module 72 includes at least two currentsensors for a three-wire system. As the three phase currents add tozero, the third current may be calculated from the other two currentvalues. However, while a third sensor is optional, such sensor increasesthe accuracy of the overall current calculation.

Control module 72 also includes an internal motor parameters calculator96, which calculates/estimates a set of reference motor parameters 98,such as, for example, rms voltage, rms current, slip (or speed), powerfactor, and efficiency, to be input to control algorithm module 88.Motor nameplate information (NPI) 100 is obtained from motor drive 68through communications or inputted by a user in control module 72. Afrequency reference 102 is also input to external control module 72through drive communications.

Similar to the procedure described with respect to FIG. 1, during anenergy-optimizing mode, the logic contained in control algorithm module88 essentially replaces static V/Hz curve block 76. Control algorithmmodule 88 receives frequency reference 102 and NPI 100 from motor drive68, as well as calculated/estimated reference motor parameters 98 frommotor parameters calculator 96 as inputs. Module 88 generates afrequency command 104 and a voltage command 106 using these inputs andcontrol module 72 sends these commands 104, 106 to signal generator 78.

According to this embodiment, since control algorithm module 88 islocated externally from motor drive 68, static V/Hz curve block 76 maybe kept as is, producing a set of preset voltage commands 108 andfrequency commands 110. However, these preset commands 108, 110 are notpassed to signal generator 78.

FIG. 3 illustrates a motor drive system 112 including an externalcontrol module 114 according to another embodiment of the invention.Similar to the motor drive system described with respect to FIG. 2,motor drive system 112 includes a motor drive 116, a drive control unit118, and a drive user interface 120. However, unlike the motor drivesystem of FIG. 2, external module 114 does not have its own voltage andcurrent sensors or an internal motor parameter calculator. Instead,external module 114 obtains a frequency reference 122, a NPI 124, andcalculated and/or estimated motor parameters 126 through drivecommunications. According to one embodiment, external module 114 may beimplemented in an extension card slot of motor drive 116 to provideenergy optimizing functionality to motor drive 116.

Referring now to FIG. 4, a controller implemented, energy-optimizingtechnique 128 for dynamically controlling a motor drive, such as ACmotor drive 12 of FIG. 1, is set forth according to an embodiment of theinvention. As described in detail below, technique 128 dynamicallyadjusts voltage and frequency applied to the motor based on motor orload demand, which effectively adjusts the shape or profile of thepre-set V/Hz curve internal to motor drive 12. Technique 128 monitorsthe value of one or more motor parameters until the motor parameter(s)fall within a pre-defined tolerance zone. While adjusting voltage andfrequency, technique 128 also monitors for abrupt load changes or motorinstabilities or undesirable operations, which may signify potentialmotor failure or undesirable motor operation.

Technique 128 begins at STEP 130 by obtaining motor nameplate parameters(NPI), such as, for example, rated horse power (HP), full load amperes(FLA), rated voltage, rated speed (RPM), rated frequency (Hz), ratedpower factor, and rated efficiency. At STEP 130, user configurationparameters are also obtained, which may include a given percentage ofFLA, a user-defined slip boundary, for example. At STEP 132, technique128 defines a set of boundaries for selected reference motor parameters,including, for example, voltage, current, speed (slip), power factor,and efficiency. For example, a voltage boundary condition, V_bound, maybe defined as a percentage of a quadratic V/Hz curve (e.g., 80%); acurrent boundary, I_bound, may be defined as a percentage of FLA; a slipboundary, s_bound, may be defined to correspond to a rated slip at arated frequency or be user-defined; and a maximum change in current,delta_I_bound, may be defined as a percentage of FLA (e.g., 20%).

At STEP 134, a set of tolerance zones are defined for selected motorparameters for determining optimal operation. Such tolerance zones mayinclude a voltage tolerance zone, a current tolerance zone, a speed(slip) tolerance zone, a power factor tolerance zone, and/or anefficiency tolerance zone, for example. According to an embodiment ofthe invention, a tolerance zone may be determined based on a definedboundary condition according to:Tolerance Zone=x*Boundary_Condition  (Eqn. 1),where x represents a selected percentage range. Using Eqn. 1, a voltagetolerance zone may be defined as V_tol_zone=[100%-105%]*V_bound, forexample. Likewise, a current tolerance zone, I_tol_zone and a sliptolerance zone, s_tol_zone, may be defined asI_tol_zone=[95%-100%]*I_bound and s_tol_zone=[95%-100%]*s_bound,respectively.

At STEP 136, technique 128 supplies an initial, default, or start-up setof voltage-frequency commands to a command generator. Default orstart-up set of voltage-frequency commands may be, for example, based onvoltage-frequency commands of a pre-set or static V/Hz curve or savedvoltage-frequency commands from a previous successful start-up event.

Technique 128 obtains a frequency reference from a user or an outer loopprocess controller of the motor drive at STEP 138. At STEP 140,technique 128 determines if the change of frequency reference issignificant enough for the drive control to respond. Specifically,technique 128 may compare a current frequency reference to a previousfrequency reference value to determine if the current frequencyreference is identical to the previous frequency reference.Alternatively, technique 128 may determine if the difference between thetwo values is greater than a preset tolerance value (e.g., 0.1 Hz). Ifthe frequency reference is changed (or the change is greater than thepreset tolerance value) 142, technique 128 applies a new referencefrequency and a new voltage from the original V/Hz setting, a linearV/Hz setting, or other pre-defined value/setting at this frequency asfrequency and voltage commands at STEP 144. Alternatively, if thefrequency reference has changed by a small amount (e.g., <0.1 Hz) thevoltage command may remain unchanged while changing the frequencycommand. Technique 128 then proceeds to STEP 146 and calculates orestimates one or more real-time motor parameters, which may correspondto a motor voltage, a motor current, a motor speed, a motor powerfactor, and/or a motor efficiency, for example.

Referring back to STEP 140, if the frequency reference is unchanged (orthe change is less than a preset tolerance value) 148, technique 128proceeds directly to STEP 146 and calculates or estimates real-timemotor parameters using a current set of voltage and frequency commands.At STEP 150, technique 128 monitors for a motor transient status, suchas, for example, an abrupt load change. If no transient status isdetected 152, technique 128 monitors for an undesirable motor operationcondition at STEP 154, which may be a detected motor current beyond acurrent boundary or a detected motor voltage beyond a voltage boundary,for example. If no undesirable motor operation condition is detected156, technique 128 monitors for an unstable operation condition at STEP158. To determine if an unstable operating condition exists, technique128 may analyze the monitored real-time line voltage and current, achange in value between reference line voltage and/or current andreal-time line voltage and/or current measurements, one or morereal-time motor parameters values, and/or the motor parameter trend todetermine an unstable operation condition. An unstable operationcondition may be detected if a motor slip (or speed) is beyond a slip(or speed) boundary, for example. Also, an unstable operation conditionmay be reflected by an abrupt change in line voltage or current, anabrupt change in a motor parameter, or by the value of the real-timemotor parameter, such as, for example, a motor temperature, torque,slip, power factor, or efficiency outside a threshold.

If an unstable operation condition is not detected 160, technique 128determines if optimal operation is reached at STEP 162 by determining ifany one of the selected reference parameters is within its definedtolerance zone, as defined at STEP 136. If no selected referenceparameter is within its defined tolerance zone 164, technique 128adjusts the voltage command at a fixed or variable step at STEP 166. Forexample, technique 128 may reduce the voltage command by a preset pulsevalue, thereby causing a modified switching signal to be transmitted tothe inverter. In one embodiment, the technique 128 increments thevoltage command by a pulse value of −10 volts. That is, the modifiedvoltage command is 10 volts lower than the default voltage command.Alternatively, if the motor drive is operating at an optimal operationpoint 168 (i.e., a motor parameter is within its tolerance zone),technique 128 maintains the current voltage command and frequencycommand. At STEP 170, technique 128 enters a wait step, whereintechnique 128 waits for an algorithm execution period before returningto STEP 138. Technique 128 then continues cycling through STEPS 138 and178 to monitor for changes in motor status.

Referring back to STEPS 150, 154, and 158, if technique 128 detects anyone of a motor transient status 172, an undesirable motor operationcondition 174, or an unstable motor operation condition 176, technique128 enters a master reset subroutine at STEP 178. During the masterreset subroutine, the frequency command remains unchanged and within afew steps (i.e., algorithm fast execution periods), the voltage commandis increased to a linear V/Hz setting or an original V/Hz setting.Alternatively, master reset subroutine may selectively increment thevoltage-frequency command to reach the stable condition by backtrackingto the last stable condition or attempting to correct the overshoot bydecreasing the previously used increment or pulse value of thevoltage-frequency command. For example, if the previous pulse value was−10 volts, technique 128 may increase the previously incrementedvoltage-frequency command by +5 volts to reach a stable point betweenthe two most recent increments of the voltage-frequency command. Once astable condition is achieved, technique returns to STEP 138.

According to one embodiment, preset execution periods may be assigned toeach step of technique 128. For example, each of STEPS 138-178 may beassigned to one of a fast execution period, Tjast, used for monitor,protection, and master reset (e.g., 0.5 seconds) and a slow executionperiod, T_slow, used for a drive command update period to update theoperation of the drive and motor (e.g., 10 seconds). It is contemplatedthat a default value for T_fast and T_slow may vary depending on thetime constant of the motor and load.

FIG. 5 shows a series of graphs of real-time motor parameters determinedusing a control technique such as that described with respect to FIGS.1-4 for a motor having a rated power of approximately 125 hp, whichcorresponds to a full voltage of approximately 268.4 V of a static andpre-set linear V/Hz curve with a 35 Hz frequency command. FIG. 5illustrates motor power factor 180, input real power 182, motor current184, and motor slip 186 as a function of voltage. As shown, input realpower 182 follows an approximately linear path as voltage is decreasedfrom rated power while keeping the frequency command constant at 35 Hz.Motor power factor 180, however, follows an approximately quadraticcurve. A tolerance zone 188 is defined corresponding to a desired rangefor a motor parameter, such as, for example, current. As shown in FIG.5, at a given operation point 190 within tolerance zone 188 the motorinput real power 182 is reduced to an optimal operating power 192 ofapproximately 14 kW by reducing voltage from full voltage of the pre-setV/Hz curve to a voltage of 168 V corresponding to operation point 190.Thus, by decreasing voltage to reach operating point 190, real power 182is reduced.

While additional energy savings may be possible by reducing the voltageto an operation point outside tolerance zone 188, such a decrease mayhave negative effects on motor stability if voltage is reduced too much.As shown in FIG. 5, at voltage values less than approximately 168 V,slip 186 increases rapidly. A slip 186 greater than approximately 3% maybe undesirable and can cause motor instability. Therefore, tolerancezone 188 may be selected as an optimal operating point since it ensuresthat slip 186 stays below the pre-defined slip boundary of 3%. Also, atvoltages beyond tolerance zone 188, current 182 increases as well,potentially leading to an over-current condition.

FIG. 6 illustrates real-time energy savings 194 achieved using thedynamic energy-optimizing control strategy set forth with respect toFIG. 4. Load characteristic curve 196 is a characteristic torque/speedcurve that describes a certain load, such as, for example, a pump. Curve198 is a torque/speed curve of a traditional motor drive operated at anoperating point with a given frequency command f_(cmd) according to astatic and pre-set V/Hz curve. According to one embodiment, frequencycommand f_(cmd) may be equal to a frequency reference f_(ref), which istypically given to the drive by the user or an outer-loop controller.The drive controls the motor to have an actual fundamental frequency off₁ that closely tracks the frequency command f_(cmd). The actualfundamental frequency f₁ in the motor determines the motor synchronousspeed ω_(syn), by:

$\begin{matrix}{{\omega_{syn} = \frac{120f_{1}}{p}},} & \left( {{Eqn}.\mspace{14mu} 3} \right)\end{matrix}$where p is the number of poles. Synchronous speed is a constant valuefor a certain motor at a fixed frequency. It is an imaginary maximumpossible speed that the motor could reach under absolute no-loadconditions, or when torque is zero.) The intersection point 200 betweencurve 196 and curve 198 determines the actual operating point of themotor under this condition, with a motor speed ω₁ 202 and an outputtorque T₁ 204.

Curve 206 is a torque/speed curve of an optimized motor drive operatedwithin a tolerance zone at an operating point with the same frequencycommand f_(cmd) (as results, same actual frequency f₁ and synchronousspeed ω_(syn)), such as operating point 188 of FIG. 5. The intersectionpoint 208 between curve 206 and curve 196 determines the actualoperating point of the motor under this optimal operating condition,with a motor speed ω₂ 210 and an output torque T₂ 212.

As shown in FIG. 6, at a given frequency command f_(cmd), thetraditional motor drive operates at torque 204. The optimized motordrive, on the other hand, operates at a lower torque 212. This decreasein torque from operation point 200 to operation point 208 results inreduced input power 194 and therefore energy savings. It is noticeablethat as the optimizing control is applied, the actual motor speedtypically drops slightly depending on the torque/speed characteristiccurve of a specific load. This is typically acceptable for applicationswhere precise speed control is not needed, such as pump, compressors,and fans.

According to one embodiment of the invention, energy savings 194 may becalculated based on a comparison of the initial voltage-frequencycommand and the modified voltage-frequency command corresponding tooperation within the tolerance zone of the motor parameter, anddisplayed as a digital power savings value on a motor drive, such asmotor drive 12 of FIG. 1, for example. The displayed energy savings mayindicate either an instantaneous energy savings and/or a cumulativeenergy savings over a pre-set time period or during the lifetime of thedrive. Also, energy savings 194 may be used to calculate a carbonreduction resulting from the decreased energy usage, which may bedisplayed on a carbon reduction meter on motor drive 12.

Referring now to FIG. 7, an optimized torque speed curve is illustratedfor applications where precise speed control is desired according to anembodiment of the invention. Load characteristic curve 214 is acharacteristic torque/speed curve that describes a certain load, suchas, for example, a pump. A first motor characteristic curve 216represents a torque/speed curve of a traditional motor drive operated atan operating point with a given frequency command f_(cmd) according to astatic and pre-set V/Hz curve 218. A second motor characteristic curve220 is a torque/speed curve of an optimized motor drive. As shown inFIG. 7, curves 216, 220 intersect load characteristic curve 214 at anintersection point 222 having a common speed and torque.

This intersection point 222 is a result of operating a motor drive in anapplication where precise speed control is desired. In such anapplication, an outer-loop process controller is typically included toadjust the frequency command and, together with energy optimizingcontrol method described with respect to FIG. 4, settle the motoroperating point to the intersection point 222 of curves 214 and 218. Inthis example, the outer-loop controller will slightly increase thefrequency command f_(cmd) so that it is slightly greater than thefrequency reference f_(ref) (in turn, the synchronous speed, fromω_(syn1) to ω_(syn2)) and eventually settle down to the originaloperating point at speed ω₁ and output torque T₁. The output power P2will be equal to the original output power P1.

However, because the second motor characteristic curve 220 is associatedwith lower voltage applied to the motor, the motor core losses will bereduced. Therefore, energy saving are achieved from the motor core lossreduction at a reduced voltage. A stable operation is achievable withthe interactions between the energy-optimizing algorithm and theexternal process controller, because in this system, the outer-loopprocess control time constant (in tens of seconds or minutes) is atleast 10 times faster than the time constant of the energy-optimizingalgorithm (in seconds).

FIGS. 8-10 show a comparison of motor voltage (FIG. 8), motor inputcurrent (FIG. 9), and motor input real power (FIG. 10) measured underfour different motor drive control conditions using a 50 hp open-loopmotor drive at 40 Hz operation: a Linear V/Hz setting 224; a QuadraticV/Hz setting 226; a Flux Minimization setting 228; and an energyoptimization setting 230, such as that described with respect to FIG. 4.As shown in FIGS. 8-10, the Flux Minimization setting 228 gives theminimal current and the energy optimization setting 230 gives theminimal input real power, which leads to maximum energy savings.

While several embodiments of the invention are described with respect toan AC motor and AC motor drive, it is contemplated that theenergy-optimizing technique set forth herein may be applied to a widevariety of applications. For example, the energy-optimizing techniquemay be used in hybrid vehicles to minimize power output or draw from abattery system or in an uninterruptible power supply (UPS) with avariable load, such as for lighting systems. The technique may also beused in any application that uses a PWM inverter, such as, for example,semiconductor applications having PWM power converters or in generalinverter applications to change a switching algorithm. The technique canbe used for a variety of voltage levels, including low-voltage,medium-voltage and high-voltage applications.

A technical contribution for the disclosed method and apparatus is thatit provides for a control unit-implemented technique for modifying avoltage-frequency command of an AC motor drive. Based on a modifiedvoltage-frequency command, the technique controls switching time of aseries of switches in a motor control device to reduce a motor torqueand a motor input power.

Therefore, according to one embodiment of the present invention, acontrol system is programmed to input an initial voltage-frequencycommand to the AC motor drive based on an initial voltage/frequency(V/Hz) curve, receive a real-time output of the AC motor drive generatedaccording to the initial voltage-frequency command, and feedback aplurality of modified voltage-frequency commands to the AC motor drive,each of the plurality of modified voltage-frequency commands comprisinga deviation from the initial V/Hz curve. The control system is alsoprogrammed to determine a real-time value of the motor parametercorresponding to each of the plurality of modified voltage-frequencycommands and feedback a modified voltage-frequency command to the ACmotor drive so that the real-time value of the motor parameter is withina motor parameter tolerance range.

The control system for controlling an AC motor drive can be viewed ashaving units (virtual) to perform or conduct the aforementioned acts ofa processor. For example, the control system comprises a unit to inputan initial voltage-frequency command to the AC motor drive based on aninitial voltage/frequency (V/Hz) curve, and a unit to receive areal-time output of the AC motor drive generated according to theinitial voltage-frequency command. The control system also comprises aunit to feedback a plurality of modified voltage-frequency commands tothe AC motor drive, each of the plurality of modified voltage-frequencycommands comprising a deviation from the initial V/Hz curve, a unit todetermine a real-time value of the motor parameter corresponding to eachof the plurality of modified voltage-frequency commands, and a unit tofeedback a modified voltage-frequency command to the AC motor drive sothat the real-time value of the motor parameter is within a motorparameter tolerance range.

According to another embodiment of the present invention, a method forcontrolling a motor drive output includes operating a motor driveaccording to a static voltage-frequency profile to generate an outputpower and determining a value of a specified motor parametercorresponding to the static voltage-frequency profile. The method alsoincludes determining a tolerance zone of the specified motor parameter,selectively modifying the static voltage-frequency profile duringoperation of the motor drive to generate a reduced motor input power,and operating the motor drive at a voltage-frequency settingcorresponding to a value of the specified motor parameter within thetolerance zone.

According to yet another embodiment of the present invention, a motordrive is configured to supply power to a load. The motor drive includesan inverter designed to provide power to the load and a controlleroperationally connected to control operation of the inverter. Thecontroller is configured to cause the inverter to adjust avoltage-frequency setting during operation of the motor drive to causethe inverter to operate according to each of a plurality of variedvoltage-frequency settings and monitor a real-time value of a motorparameter corresponding to each voltage-frequency setting. Thecontroller is also configured to identify a tolerance zone for the motorparameter and cause the inverter to operate according to thevoltage-frequency setting such that the motor parameter is within thetolerance zone.

The present invention has been described in terms of the preferredembodiment, and it is recognized that equivalents, alternatives, andmodifications, aside from those expressly stated, are possible andwithin the scope of the appending claims.

1. A control system for controlling an AC motor drive, the controlsystem programmed to: input an initial voltage-frequency command to theAC motor drive based on an initial voltage/frequency (V/Hz) curve;receive a real-time output of the AC motor drive generated according tothe initial voltage-frequency command; feedback a plurality of modifiedvoltage-frequency commands to the AC motor drive, each of the pluralityof modified voltage-frequency commands comprising a deviation from theinitial V/Hz curve; determine a real-time value of the motor parametercorresponding to each of the plurality of modified voltage-frequencycommands; and feedback a modified voltage-frequency command to the ACmotor drive so that the real-time value of the motor parameter is withina motor parameter tolerance range; wherein the modifiedvoltage-frequency command operates the AC motor drive at a reduced inputpower as compared to the initial voltage-frequency command, whilemaintaining the real-time value of the motor parameter within the motorparameter tolerance range.
 2. The control system of claim 1 wherein themotor parameter comprises one of a motor rms voltage, a motor rmscurrent, a motor input power, a motor speed, a motor slip, a motor powerfactor, a motor efficiency, and a motor temperature.
 3. The controlsystem of claim 1 programmed to monitor at least one of the real-timevalue of the motor parameter and a rate-of-change of the real-time valueof the motor parameter.
 4. The control system of claim 1 programmed todetermine a value of at least one of an instability identificationparameter and an undesirable operation parameter based on the receivedreal-time output of the AC motor drive, the instability identificationparameter and the undesirable operation parameter comprising at leastone of a motor voltage, a motor current, a motor torque, a motor speed,a motor slip, a motor power factor, a motor efficiency, and a motortemperature.
 5. The control system of claim 4 programmed to: detect amotor instability condition based on the value of the instabilityidentification parameter; and adjust the voltage-frequency command ifthe motor instability condition is detected.
 6. The control system ofclaim 5 programmed to reset the voltage-frequency command within aperiod to match one of the initial V/Hz curve, a linear V/Hz curve, anda pre-defined V/Hz curve if the motor instability condition is detected.7. The control system of claim 4 programmed to: detect an undesirablemotor condition based on the value of the undesirable operationparameter; and adjust the voltage-frequency command if the undesirablemotor condition is detected.
 8. The control system of claim 1 whereinthe real-time output of the AC motor drive comprises at least one of areal-time voltage and a real-time current.
 9. The control system ofclaim 8 programmed to detect a motor instability condition and atransient motor condition based on a value or a rate-of-change of the atleast one of a motor power factor, efficiency, motor slip, voltage,current, torque, and temperature.
 10. The control system of claim 1programmed to adjust the voltage-frequency command by a predefined valueto modify the voltage-frequency command.
 11. The control system of claim1 programmed to: compare the initial voltage-frequency command and themodified voltage-frequency command corresponding to the value of themotor parameter within the motor parameter tolerance zone; and determinea motor input power savings based on the comparison.
 12. A method forcontrolling a motor drive output comprising: operating a motor driveaccording to a static voltage-frequency profile to generate an outputpower; determining a value of a specified motor parameter correspondingto the static voltage-frequency profile; determining a tolerance zone ofthe specified motor parameter; selectively modifying the staticvoltage-frequency profile during operation of the motor drive togenerate a reduced motor input power; and operating the motor drive at avoltage-frequency setting corresponding to a value of the specifiedmotor parameter within the tolerance zone.
 13. The method of claim 12wherein selectively modifying the default voltage-frequency profilecomprises: adjusting a voltage-frequency setting during operation of themotor drive such that a reduced motor input power is generated accordingto a dynamic voltage-frequency profile; and determining a value of thespecified motor parameter for each voltage-frequency setting of thedynamic voltage-frequency profile.
 14. The method of claim 12 whereindetermining the value of the specified motor parameter comprisesdetermining the value of one of a motor rms voltage, a motor rmscurrent, a motor input power, a motor speed, a motor slip, a motor powerfactor, a motor efficiency, and a motor temperature.
 15. A motor driveconfigured to supply power to a load, the motor drive comprising: aninverter designed to provide power to the load; and a controlleroperationally connected to control operation of the inverter, thecontroller configured to: adjust a voltage-frequency setting duringoperation of the motor drive to cause the inverter to operate accordingto each of a plurality of varied voltage-frequency settings; monitor areal-time value of a motor parameter corresponding to eachvoltage-frequency setting; identify a tolerance zone for the motorparameter; and cause the inverter to operate according to thevoltage-frequency setting such that the motor parameter is within thetolerance zone; wherein adjusting the voltage-frequency setting duringoperation of the motor drive comprises adjusting at least one of a motorvoltage and a motor frequency, so as to minimize at least one of a motortorque and a motor input power.
 16. The motor drive of claim 15 whereinthe tolerance zone comprises a range of values of the motor parametercorresponding an optimized operation of the motor drive.
 17. The motordrive of claim 15 wherein the monitored real-time motor parametercomprises one of an rms voltage, an rms current, a motor speed, a motorslip, a motor power factor, and a motor efficiency.
 18. The motor driveof claim 15 wherein the controller is configured to: detect anundesirable motor condition based on a real-time value of at least oneof a motor power factor, efficiency, motor slip, voltage, current,torque, and temperature; and upon detection of the undesirable motorcondition, reset the voltage-frequency setting to one of an initialvoltage-frequency setting, a linear voltage-frequency setting, and apre-defined voltage-frequency setting.